Solid phosphoric acid catalysts in which phosphoric acid is supported on an inorganic support are widely used in the hydration reaction or oligomerization reaction of olefins.
On the other hand, oligomers of olefins are used in various applications and dimers of light olefins (for example, propylene, n-butene, isobutane and pentene) are particularly important as high octane number base materials for gasoline or chemical intermediate raw materials. Oligomerization reactions including dimerization of olefins are carried out using an acid catalyst and many studies have been done on them. Typical examples of acid catalysts include liquid or gas catalysts such as sulfuric acid, hydrofluoric acid, phosphoric acid, aluminum chloride and boron fluoride and solid catalysts such as amorphous or crystalline aluminosilicate, clay, ion exchange resin, composite oxide and acid supported on a support. The above-described solid phosphoric acid catalysts which can be produced by an inexpensive and easy process have also been extensively studied.
Examples of oligomerization reactions of olefin by a solid phosphoric acid catalyst proposed include a process for oligomerizing propylene using a solid phosphoric acid catalyst prepared under a calcination condition of higher than 100° C. (see, for example, patent document 1) and a process for oligomerizing propylene using a catalyst (composed of silicon orthophosphate and silicon pyrophosphate) prepared by crystallizing an amorphous mixture of phosphoric acid and a siliceous raw material at 250 to 450° C. in an atmosphere of an air-water vapor mixed gas at a water vapor concentration of 3 to 50% by mole (see, for example, patent document 2). Also, the influence of the condensation degree of phosphoric acid in a solid phosphoric acid catalyst on the activity of oligomerization reaction of olefin has been known. For example, patent document 3 and non-patent document 1 disclose a process for oligomerizing C3 or C4 olefin using a catalyst in which the weight ratio of free phosphoric acid components (non- or low-condensed phosphoric acid such as orthophosphoric acid and pyrophosphoric acid) eluted when immersing a solid phosphoric acid catalyst in water to the catalyst is small (the ratio of orthophosphoric acid to phosphoric acid supported being about 46% by mole at most in terms of phosphorus atoms).
However, none of the above conventional oligomerization reactions of olefin using a solid phosphoric acid catalyst is primarily directed to dimerization of olefin, and by-production of a highly polymerized product of olefin was inevitable when using a conventional solid phosphoric acid catalyst, making selective production of olefin dimer difficult.
There are studies on improvement of the selectivity of dimerization of olefin; for example, patent document 4 proposes a process for dimerizing olefin using a solid phosphoric acid catalyst in which the ratio of orthophosphoric acid to phosphoric acid supported on a support is 60% by mole or more in terms of phosphorus atoms.
On the other hand, it is unknown that in oligomerization reaction of olefin using a solid phosphoric acid catalyst, a reaction solution obtained at the outlet of a reactor contains phosphoric acid eluted from the catalyst and the amount of elution varies depending on the temperature of the oligomerization reaction. And there is no finding on controlling the amount of elution by selecting the temperature of the oligomerization reaction.
Moreover, phosphoric acid eluted into the reaction solution is concentrated when separating LPG fraction and oligomer or oligomer and heavy substances in a distillation column or the like in the downstream stage of an oligomerization reaction process, possibly causing corrosion of the apparatus. For that reason, effective removal of phosphoric acid in the reaction solution is desired.